If current trends continue, the world will likely blow past the 2 degree Celsius (3.6 degree Fahrenheit) warming barrier that scientists believe is the minimum for safety.

"If you look at the commitments today from governments around the world, we've only got a 50-50 shot at a 3 C (5.4 F) world, almost no chance of a 2 C (3.6 F) world, and to be quite honest I would say it's not unlikely that we will hit a 5 C (9.0 F) world," Bob Watson, former head of the UN's climate panel and chief advisor to Britain's environment ministry, told the AFP.

Rising temperatures would trigger changes such as the melting of ice sheets and massive tree loss from droughts that cannot be reversed.

The four-day conference will discuss the planet's health in a new geological era shaped by humans.